mysql -- ALTER MERGE denial of service vulnerability

Dean Ellis reported a denial of service vulnerability in the MySQL
server:

Multiple threads ALTERing the same (or different) MERGE tables to
change the UNION eventually crash the server or hang the individual
threads.

Note that a script demonstrating the problem is included in the
MySQL bug report. Attackers that have control of a MySQL account can
easily use a modified version of that script during an attack.